The present invention relates to an optical demultiplexer and particularly to an optical demultiplexer used for wavelength multiplexing optical communication.
In an optical demultiplexer used for wavelength multiplexing optical communication in the related art, photo acceptance elements or the like are arranged at wavelength intervals of Δλ in accordance with channels standardized by the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T). Parallel light rays incident onto a diffraction grating are decomposed into parallel pencils of rays having different diffraction angles in accordance with wavelengths by the wavelength dispersion function of the diffraction grating. The parallel pencils of rays are converged and image-formed into different positions in accordance with wavelengths by a light collimating element such as a lens. The photo acceptance elements are arranged in the converging/image-forming positions respectively, so that optical signals corresponding to channels are output from the photo acceptance elements respectively.
On this occasion, it is necessary to make the pitch ΔS between the converging/image-forming positions coincident with the pitch ΔP between the photo acceptance elements in order to obtain the function of the optical demultiplexer used for wavelength multiplexing optical communication. Generally, the focal length f of the light collimating element is decided on the basis of the following expression (1) of reciprocal linear dispersion of the diffraction grating so that the converging/image-forming pitch can be obtained to be matched with the photo acceptance element pitch.D=cos β/Nmf=Δλ/ΔS=Δλ/ΔP  (1) in which D is the quantity of wavelength dispersion (reciprocal linear dispersion) per unit length, β is the angle of diffraction, N is the number of grooves in the diffraction grating, and m is the order of diffraction.
In the expression (1), the diffraction angle β is decided by substituting the relation λ=λc (λc is the wavelength used) and the angle α incident onto the diffraction grating into the following expression (2) of the diffraction grating.sin α+sin β=Nmλ  (2) 
Generally, when the diffraction angle β in a wavelength λc used optionally is small, cos β in the expression (1) is approximately equal to 1 (constant). Hence, even in the case where the used wavelength λc changes in accordance with the change of the used channel so that the diffraction angle β changes on the basis of the expression (2), Δλ/ΔS becomes constant because the left side (cos β/Nmf) of the expression (1) is constant. Hence, the design method in the related art has no practical problem in this condition. (See Japanese patent publication P2002-50778A, and U.S. patent application Ser. No. 09/915,477.)
On the other hand, in the demultiplexer used for narrow-band wavelength multiplexing optical communication, it is necessary to make the dispersion performance of the diffraction grating high. Hence, it is desirable that the number of grooves in the diffraction grating is increased so that the demultiplexer is used in a place where sinβ takes a large value. When the demultiplexer is used in this condition, the reciprocal linear dispersion D, however, varies in accordance with the used wavelength λc because cos β is not approximately equal to 1. As a result, the pitch ΔS between the converging/image-forming positions changes. Hence, there is a problem that optical signals of respective channels cannot be accepted by the photo acceptance elements if the pitch ΔP between the photo acceptance elements is not changed in accordance with the change of ΔS.
As measures to solve this problem, there may be conceived a method using photo acceptance elements arranged at such irregular pitches as follows. That is, the pitch ΔP between the photo acceptance elements is changed in such a manner that the converging/image-forming position pitch ΔS between adjacent channels is corrected on the basis of the reciprocal linear dispersion D calculated by the expression (1) in accordance with each channel. This method is however based on linear calculation using cosβ as a constant for adjacent channels even in the case where the wavelength λ changes from λc to λc+Δλ so that the diffraction angle β changes from βc to βc+Δβ. For this reason, when the dispersion performance of the diffraction grating needs to be made high as described above, Δβ increases with increase in number of grooves. In this case, if the diffraction angle is taken large, βc becomes large. As a result, there is a problem that unacceptable error occurs in a narrow-band multi-channel optical demultiplexer.
The invention is provided in consideration of the problems in the related art. An object of the invention is to provide an optical demultiplexer in which converging/image-forming positions of channels are made coincident with positions of photo acceptance elements respectively so that respective signals of the channels can be taken out from the photo acceptance elements efficiently and accurately even in the case where the optical demultiplexer is a narrow-band multi-channel optical demultiplexer.